Hi Gyhoket, sorry you're having issues with it.
Can you try right-clicking the hierarchy and checking if that works instead? If that doesn't work either then make sure you've imported all the assets in the package, and that _EditorMeshTools.cs is in your Editor folder under Assets/Editor/MeshEdit.
It looks like the option on the menu bar has disappeared so I'll look into a fix. Thanks for letting me know!